Materials made of titanium aluminides (TiAl) are considered to be unsuitable for fusion welding and, therefore, also as unsuitable for build-up welding.
German Patent Publication DE 60 2006 000 955 T2 describes a method for repairing turbine blades made of, for example, Ti17 or RA6V by laser powder build-up welding, where a powder material which is of the same kind, but not necessarily identical, and which has a grain size of preferably between 30 and 100 μm is delivered through a nozzle that is concentric with the laser beam.
A method for fabricating articles by deposition of successive layers is known from German Patent Publication DE 39 42 051 A1. The method is exemplified for complete build-up or repair of a compressor blade. In this process, a substrate is surface-melted by a laser beam in an inert gas atmosphere, and a preheated powder is fed laterally to the melted region using an inert carrier gas and incorporated by fusion. After solidification of the melt, the process is repeated to deposit the next layer. The layer thickness is, for example, 0.38 mm, and Ti-6Al-4V (also known as RA6V) is mentioned as a material for the substrate and the powder.
However, the difficulties in build-up welding of titanium aluminides persist. Due to the high melting temperature of TiAl, it is difficult to suitably control the temperature profile and to manage the thermal stresses in the component. This problem occurs in particular, but not exclusively, in repair and hardfacing processes where only a localized region of a component is thermally affected. It is particularly difficult to perform build-up welding near abrupt, in particular acute-angle changes in the contour of, for example, notches and the like. Examples include Z-notches of turbine blades, where a flank of the notch is hardfaced near the notch radius so that it will be able to resist the high mechanical stresses. Information on the function and an exemplary shape of a Z-notch on a rotor blade tip shroud of a turbine stage can be found, for example, in German Patent Publication DE 10 2008 037 521 A1.
United States Patent Publication US 2005/0067064 A1 describes a laser build-up welding method for depositing a steel powder alone or a mixture thereof with TiC—, TIN- or WC powder on a steel substrate. The process is a two-stage process where first the powder material is placed in a layer having a thickness of 0.08″ and a width of 0.2″ and is then incorporated by fusion using a laser beam. A similar method for coating a substrate of an aluminum alloy with a TiC powder alone or in a mixture with Si and/or Cu is disclosed in U.S. Pat. No. 4,732,778.